The present invention relates to medical imaging systems such as X-ray equipment, computed tomography imaging apparatus and magnetic resonance imagers, and more particularly, to techniques for removing image artifacts that are produced by movement of the patient.
For example, in a computed tomography system an X-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane". The X-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the X-ray beam by the patient and each detector produces a separate electrical signal that is a measurement of beam attenuation along a specific ray path. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a common type of CT system are rotated on a gantry within the imaging plane and around the patient so that the angle at which the X-ray beam intersects the patient constantly changes. A group of X-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the patient comprises a set of views made at different angular orientations during one revolution of the X-ray source and detector. The gantry may stop or continue to move as the measurements are being made. The image produced from the scan data correspond to a two dimensional slice taken through the patient.
Typical CT systems may be operated in either the axial mode or the helical scan mode. In the typical axial mode, the patient being imaged remains stationary during each scan and the gantry revolves once to complete a scan. The gantry may make additional revolutions to acquire image data at the same slice position through the patient in order to observe temporal changes, such as occur in the heart at different stages of the cardiac cycle. The patient may also be moved between rotations in order to observe different slices through the patient. In the conventional helical scan mode, the gantry with the X-ray source and detector array revolves continuously while the patient is translated through the imaging plane. Each revolution of the gantry, or scan, acquires projection data through the patient in three dimensions in a single operation. These data are processed subsequently to form the desired image plane through the patient.
The resultant set of projections from a scan are used to reconstruct an image which reveals the anatomical structures at the position of each slice taken through the patient. The prevailing method for image reconstruction image is referred to in the art as the filtered back-projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
It is well recognized that artifacts caused by patient movement are limiting factors in the imaging of small structures in the chest and abdomen. Normal motion of the chest wall and diaphragm during breathing causes movement of internal organs during the scan interval. Such movement changes the shape of the imaged object during the scan and thereby produces blurring, streaking, dark bands and voids, and anatomical distortion in the reconstructed image.
Presently it is a common clinical practice to request a patient to hold his or her breath during the CT scan to alleviate motion artifacts. Although this can be done by a cooperative patient, it may not be possible in subjects with serious disease who are incapable of holding their breath for all but the shortest durations. A voluntary breath hold is out of the question for pediatric procedures, or when the patient is uncooperative or unconscious as in trauma cases.
A technique for acquiring motion-free images from uncooperative patients is to employ physiological gating of the imaging scan. One such approach is described in U.S. Pat. No. 4,994,965 in which the image acquisition interval is centered within a quiescent period of the periodic motion. Quiescent periods occur during the respiratory cycle at the ends of expiration and inspiration. The chest anatomy is well separated in an image acquired at the end of inspiration, whereas the end of expiration is preferred for abdominal imaging. Methods, such as the one disclosed in the above-referenced U.S. patent, have been devised to predict the approach of the quiescent period for the purpose of initiating the image scan. However, these prior prediction methods assumed that respiration was a periodic function.
The present inventors through experimentation have determined that human respiration is not periodic and can vary markedly over several breathing cycles. In addition, the amplitude of lung expansion and therefore the motion of other organs varies significantly from breath to breath. Thus, techniques based on the assumption that respiration is a periodic physiological function or based on the magnitude of patient movement may not always gate image acquisition at the optimum point in time.